The present invention describes materials and methods for achieving alignment of liquid crystal materials on a substrate surface and devices fabricated using these methods and materials.
Liquid crystal display devices (LCDs) or light shutters generally comprise a layer of liquid crystalline material between two solid substrates to form a cell. These substrates are generally coated with a conducting material, such as Indium/Tin Oxide (ITO) to form electrodes or electrode patterns. An electric field applied across the cell or between the electrodes switches the liquid crystal between different molecular arrangements or states. Thus the light transmission through the cell can be modulated depending on the cell configuration, the type of liquid crystalline material, the presence of polarisers, etc. A preferred molecular alignment direction and pretilt angle (xcex8) is imparted by an alignment layer on top of the electrodes and in contact with the liquid crystalline material.
It is well known in the art that fabrication of liquid crystal devices which have advantageous performance and low defect densities requires control of the alignment of the liquid crystalline material at the surfaces of the device. Different types of liquid crystal alignment have been described. Homeotropic alignment refers to an alignment in which the unique optical axis of a liquid crystal phase is held perpendicular to the adjacent surface.
Planar alignment, sometimes referred to as homogeneous alignment, refers to alignment in which the unique optic axis of the liquid crystal phase lies parallel to the adjacent surface. Planar alignment may also impose a direction in which the optic axis of the liquid crystal lies in the plane of the adjacent surface.
Tilted planar alignment or tilted homogeneous alignment refer to alignment in which the liquid crystal unique optic axis lies at an angle, termed the pretilt angle (xcex8) from the plane of the adjacent surface. The pretilt angle may be as small as a fraction of one degree or as large as several tens of degrees.
Tilted homeotropic alignment refers to an alignment in which the optic axis of the liquid crystal lies tilted away from the normal to the adjacent surface. This deviation is again termed a pretilt angle.
In liquid crystal devices, said alignment geometries are chosen and used in combination to achieve specific optical and electro-optic properties from the device and may be combined in new ways or with new liquid crystalline mixtures to provide new types of devices.
Several methods are known in the art by which defined liquid crystal alignment may be achieved. Deposition of a polymer layer, for example a polyimide layer, on the substrate surface followed by mechanical rubbing provides a pretilted planar alignment. A planar alignment or tilted planar alignment may also be achieved by evaporating a variety of inorganic substances, for example SiOx, onto the surface from an oblique angle of incidence. A disadvantage of this method is that it requires slow and costly vacuum processing. A further disadvantage is that the resulting evaporated layer may show a high capacity to absorb contaminants onto itself from the environment or from other materials used in fabrication of the device.
A homeotropic alignment can be obtained by depositing a surfactant, for example a quatemary ammonium salt, onto the surface from solution in a suitable solvent. A disadvantage of this treatment is that the resistivity of the liquid crystal device may be lowered by the surfactant and the resulting alignment may also show poor stability.
Structured alignment patterns of subpixel size and above can be achieved by illumination of a polymer layer containing photochemically orientable dyes or photochemically dimerisable and/or isomerisable molecules, as described, for example, in EP-A-0445629. A disadvantage of this method is that the solubility of the dye molecules in the polymer matrix is limited and the chemical and photochemical stability over time is insufficient.
Another method for achieving structured non-contact orientation is the photodimerisation of polymers incorporating photodimerisable groups, such as cinnamate or coumarin derivatives, as described, for example, in Jpn. J. Appl. Phys., Vol., 31, 2155 (1995) and EP-A-9410699.0. A disadvantage of these materials is the polydispersity of the materials produced by polymerisation. This requires, for example, different solution concentrations for spin coating depending on the average molecular weights of the polymers which can not be determined with any great accuracy and which are often not reproducible from one batch to another. This can give rise to unreproducible alignment as well as also requiring repeated purification cycles of the polymer product in order to remove unreacted monomer and oligomers. The attachment of low molar mass photoreactive units to monodispersed polymer backbones can lead to polymers with unreacted sites, which can give rise to dielectric breakdown of cells containing such materials. This is especially important for active matrix devices.
An object of this invention is to provide means of achieving a defined surface alignment of a liquid crystalline material on a substrate surface, which does not require mechanical rubbing or other methods of physical contact which may damage the surface or structures on the surface. This is especially important for active matrix displays based on the use of surface mounted thin film transistors. Static electricity or dust caused by mechanical rubbing or buffing polymer layers, such as polyimide or polyamide, in order to induce a unidirectional alignment due to microgrooves can cause defects in thin film transistors and lead to dielectric breakdown. Such alignment layers also suffer from the disadvantage that the microgrooves possess inherent defects themselves, which can result in random phase distortion and light scattering. This impacts detrimentally on the optical appearance of the displays or the efficiency of the light shutters. Additionally, mechanical buffing does not allow locally oriented regions of the surface to be aligned with different azimuthal angles. This is a substantial drawback since sub-pixelisation can lead to higher contrast and an improved optical efficiency.
According to this invention materials are provided of Formula I: 
where
X1-8 are each independently selected from: H, halogen, CN, OH, straight or branched chain alkyl having from 1 to 16 carbon atoms, where one or more non-adjacent CH2 groups may be substituted by CH(CN), CH(CF3), CHF, CHCl, CH(CH3);
S1-8 are spacer units;
PG1-4 are photopolymerisable/dimerisable groups
m1, m2, m3, and m4 are each independently selected from the integers 1 and 0;
A1-8 are each independently selected from the aromatic rings: 
where xcx9c indicates a sigma bond between part of the molecule shown in formula I and a carbon atom at any position in one of the aromatic rings;
and where the CH groups present in the aromatic rings may each be independently substituted by C(CN), C(CF3), C-halogen, C(CH3), CR, where R is selected from straight or branched chain alkyl and may include from 1 to 8 carbon atoms and including where one or more non-adjacent CH2 groups may be substituted by CH(CN), CH(CF3), CHF, CHCl, CH(CH3).
Preferably the spacer groups S1-4 are, independently of one another, selected from groups having the general formula:
L1xe2x80x94(CH2)nxe2x80x94L2
where: n=1 to 30, where each CH2 group present in the chain linking L1 and L2 may be independently substituted by CH(CN), CH(CF3), CHF, CHCl, CH(CH3), L1 and L2 are independently selected from: single covalent bond, O, COO, OOC, CH2O, and OCH2. More preferably S1-4 are independently selected from oxycarbonylalkanoyloxy, oxyalkoxy, oxycarbonylalkoxy, oxyalkanoyloxy, oxycarbonylphenoxyalkanoyloxy, oxyalkoxyalkyl containing from 1-16 carbon atoms.
In a preferred embodiment spacer groups S5-8 are each independently selected from: COO, OOC, Cxe2x89xa1C, Cxe2x95x90C, single covalent bond.
Preferably the photopolymerisable/dimerisable groups PG1-4 are each independently selected from: 
where a sigma bond exists between part of the molecule shown in formula I and any one of the four C atoms that are in the benzene ring to which G is fused and that do not form part of the ring G; and where CH groups present in the benzene ring to which the ring G is fused may each be independently substituted by C(CN), C(CF3), C-halogen, C(CH3), CR, where R is selected from straight or branched chain alkyl and may include from 1 to 8 carbon atoms and including where one or more non-adjacent CH2 groups may be substituted by CH(CN), CH(CF3), CHF, CHCl, CH(CH3);
where G is independently selected from: 
and where J is independently selected from: 
R1 may be H, halogen, CN, NO2, NCS, SCN, alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC, CO and/or CHxe2x95x90CH;
R2 may be H or C1-10 alkyl;
D1 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl, or phenyl which may be substituted from one and up to and including all available substitution positions with one or more of the groups selected from CN, halogen, NO2;
E1 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR5;
R5 may be C1-10 alkyl.
D2 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl, or phenyl which may be substituted from one and up to and including all available substitution positions with one or more of the groups selected from CN, halogen, NO2;
E2 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR6;
R6 may be C1-10 alkyl.
D3 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl, or phenyl which may be substituted from one and up to and including all available substitution positions with one or more of the groups selected from CN, halogen, NO2;
E3 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR7;
R7 may be C1-10 alkyl.
D4 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl, or phenyl which may be substituted from one and up to and including all available substitution positions with one or more of the groups selected from CN, halogen, NO2;
E4 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR8;
R8 may be C1-10 alkyl.
D5 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl, or phenyl which may be substituted from one and up to and including all available substitution positions with one or more of the groups selected from CN, halogen, NO2;
E5 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR9;
R9 may be C1-10 alkyl.
D6 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl, or phenyl which may be substituted from one and up to and including all available substitution positions with one or more of the groups selected from CN, halogen, NO2;
E5 may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR9;
R9 may be C1-10 alkyl.
Examples of the term xe2x80x9calkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC, CO and/or CHxe2x95x90CHxe2x80x9d include in the present application straight-chain and branched (optionally chiral) residues such as alkyl, alkenyl, alkoxy, alkenyloxy, alkoxyalkyl, alkoxyalkenyl, 1-fluoroalkyl, terminal trifluoromethylalkyl, terminal difluoromethylalkyl, terminal trifluoromethylalkoxy, and the like with 1 or, 2 to 16 carbon atoms. Examples of preferred residues are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, 1-methylpropyl, 1-methylheptyl, 2-methylbutyl, 3-methyl pentyl, vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-hexenyl, 4-pentenyl, 4Z-hexenyl, 5-hexenyl, 6-heptenyl, 7-octenyl, methoxy, ethoxy, propyloxy, butyloxy, pentyloxy, hexyloxy, octyloxy, 1-methylpropyloxy, 1-methylheptyloxy, 2-methylbutyloxy, 1-fluoropropyl, 2-fluoropropyl, 2,2-difluoropropyl, 3-fluoropropyl, 3,3-difluoropropyl, 3,3,3-trifluoropropyl and the like. Especially preferred residues possess 1 or, respectively, 2 to 6 carbon atoms.
The term xe2x80x9chalogenxe2x80x9d may represent in the present application fluorine, chlorine, bromine and iodine, but especially fluorine and chlorine.
According to an aspect of this invention a method is provided for forming an alignment layer on a surface of a liquid crystal cell wall, the method comprising the steps: depositing a layer of a material comprising at least one compound of Formula I on the surface; and exposing the material to actinic radiation.
Preferably the method for forming an alignment layer further comprises the step of controlling the exposure time and/or intensity of the actinic radiation used to provide a selected value of pretilt in a liquid crystal placed in contact with the exposed layer.
Preferably the radiation includes light, with a wavelength of 250-450 nm. More preferably the radiation is light with a wavelength of 300-400 nm.
According to a further aspect of this invention a liquid crystal device comprises a layer of a liquid crystal material contained between two cell walls both carrying electrode structures and surface treated to provide an alignment layer for liquid crystal molecules;
characterised in that:
the alignment layer comprises a compound of Formula 1 that has been exposed to actinic radiation.
Preferably the alignment layer comprises a compound of Formula I that has been exposed to actinic radiation, the exposure time and/or intensity of the actinic radiation used being controlled to provide a selected value of pretilt in a liquid crystal placed in contact with the exposed layer.
Preferably the radiation includes light, with a wavelength of 250-450 nm. More preferably the radiation is light with a wavelength of 300-400 nm.
Compounds of Formula I can be prepared by various routes from commercially available starting materials. Typically tetrakis(hydroxymethyl)methane (pentaerythritol) can be esterified with xcfx89-halogenoalkanoic acids in the presence of N,N-dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine and a polar solvent, such as N,N-dimethylformamide or dichloromethane. The resultant bromides can then be alkylated in a Williams ether synthesis with phenols incorporating a photoisomerisable/dimerisable group, such as coumarin or cinnamate, in the presence of a base, such as potassium carbonate, and a polar solvent, such as cyclohexanone or ethyl-methylketone. The bromides can also be esterified with a photoisomerisable/dimerisable group, such as cinnamic acids, in the presence of DBU and a non polar solvent, such as toluene or benzene. Similarly pentaerythritol can be alkylated with xcfx89-halogenoalkanols protected, for example as the THP derivative, in the presence of base, such as potassium tert.-butylate, and a polar solvent, such as 1,2-dimethoxyethane or ethylene glycol dimethyl ether. After deprotection the resultant alcohols can then be esterified with a photoisomerisable/dimerisable group, such as aromatic acids incorporating a cinnamate or coumarin moiety, in the presence of N,N-dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine and a polar solvent, such as N,N-dimethylformamide or dichloromethane. The alcohols can also be alkylated in a Mitsunobu reaction, with a photoisomerisable/dimerisable group, such as 6-hydroxycoumarin, 7-hydroxycoumarin (umbelliferone) or alkyl hydroxycinnamates, in the presence of a dehydrating agent, such as diethyl azodicarboxylate and triphenyl phosphine, and a polar solvent, such as tetrahydrofuran or N,N-dimethylformamide.
The photocross-linkable groups, such as cinnamic acids, cinnamate esters, cinnamonitriles, styrenes, stilbenes, vinylnaphthalenes, vinylpyridines, maleimides, thymines, coumarins, are generally are either commercially available or readily accessible, for example coumarin and cinnamate derivatives can be prepared according to literature methods, such as the Perkin, Pechmann, Knoevenagel, Wittig-Homer, Heck or sigmatropic rearrangement reactions (Organic Reactions, 1, 210, 1942; Organic Reactions, 15, 204, 1967; Synthesis, 131, 1978; J. Mol. Cat., 88, L113, 1994; J. Chem. Soc. Perkin Trans. I, 1753, 1987).
In order to obtain alignment layers in regions selectively limited by area, a solution of the photoactive pentraerythritol derivative can, for example, firstly be prepared and then spread out using a spin-coating apparatus on a carrier coated with an electrode , e.g., a glass plate coated with indium-tin oxide (ITO) such that homogeneous layers of 0.05-50 xcexcm thickness result. Subsequently or simultaneously, irradiation can be applied to the region to be isomerised and/or dimerised (cross-linked), e.g., with a mercury high pressure lamp, a xenon lamp or a UV laser utilising a polariser and optionally a mask for the formation of structures. The duration and irradiation depends on the capacity of the individual lamps and can vary from a few minutes to several hours. The cross-linking can, however, also effected by irradiating the homogeneous layer using filters which, e.g., let through only radiation suitable for the cross-linking reaction. Photosensitisors, such as acetophenone or benzophenone may be added to shorten the illumination time required for cross-linking. Non-zero tilt angles (xcex8) may be induced by illumination with plane polarised light from a non-perpendicular angle to the plane of the substrate.